It was on the decline. I joined the forum post 07/08.. and I remember in the newbies how many of us were not happy with the performances we were seeing. Our defensive record in 08/09 was superb but there was no doubt that there was a huge drop in the 'entertainment' factor to the year before.
We felt like we had already peaked and the team was in need of new blood and energy (compared to the standards that had been set in 06-08). The years after it.. the team progressively got worse, losing Ronaldo and Tevez (two out and out world class players with bundles of flair/energy/workrate) was only going to result in our attack becoming more blunt. Suddenly we had Berbatov, Nani and Valencia.. a huge drop in terms of attacking quality.
In defence whereas the likes of Ferdinand and Vidic were once invincible and physically robust, they soon started picking up injuries so we had to play slightly more defensive to adapt to their weakened physical state.
Scholes who was in his prime still in 06/07, hugely influential the year after.. was clearly on the physical decline and we really needed the likes of Anderson to step up to the plate.. and he never did. Fergie never bothered rectifying the issue.
The signing of Van Persie injected some much needed quality up front but alot of posters were fearful that it was a stop gap signing and that the rest of the team needed alot of work.
It is absolutely false to say that everyone was completely blinded by our success and thought we were a side on the up even when we won our last title. I certainly didn't and I know many posters who were concerned about the state of our first team especially post his retirement.
No one thought it would get this bad though, I genuinely thought.. one bad season and a clear-out, followed by strong signings we would be fine but it hasn't been like that.
https://www.redcafe.net/threads/blessing-in-disguise-or-madness.311496/
That is me in 2010 saying maybe we should get maybe get rid of Rooney and focus on recruiting first teamers of the future like Bale/Modric. I wasn't spot on (also mention Rodwell haha) but the point stands, I could sense that the balance of the team was heavily reliant on ageing big stars such as VDS, Scholes, Giggs and the defense.. and we didn't have a spine once they were to leave.
https://www.redcafe.net/threads/do-you-enjoy-watching-this-united-side.369087/
https://www.redcafe.net/threads/are-we-the-england-national-team-in-disguise.377315/
After our CL win, we just weren't the same side to watch - still remained hugely successful because there was alot of know how within the team until the end of Fergies reign, but performance wise we clearly peaked in 2008 and it was not the same after that although the results stayed consistent. A huge drop off was inevitable and we should have been planning for the future properly but didn't.